Condensed Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]
Interest income	$ 1,708,098	$ 1,791,101	$ 1,866,664
Other income	41,800	35,742	35,453
Operating expenses	591,778	593,833	574,683
Income before income taxes	747,126	780,909	734,577
Income tax benefit	(271,579)	(279,803)	(254,540)
Net income	475,547	501,106	480,037
Parent Company
Condensed Financial Statements, Captions [Line Items]
Interest income	702	1,121	1,064
Dividends received from subsidiaries	450,000	485,000	555,000
Loss on debt redemption		(2,313)
Other income	525	1,174	753
Gross income	451,227	484,982	556,817
Operating expenses	38,268	44,651	42,185
Income before income taxes	412,959	440,331	514,632
Income tax benefit	16,547	20,029	16,445
Income before equity in undistributed (overdistributed) earnings of subsidiaries	429,506	460,360	531,077
Equity in undistributed (overdistributed) earnings of subsidiaries	46,041	40,746	(51,040)
Net income	$ 475,547	$ 501,106	$ 480,037